Home run arc detection at the photovoltaic string level using multiple current sensors

ABSTRACT

Systems and methods of detecting arcing in DC power systems that can differentiate between DC arcs and load-switching noise. The systems and methods can determine, within a plurality of predetermined time intervals, at least the pulse count (PC) per predetermined time interval, and the pulse duration (PD) per predetermined time interval, in which the PC and the PD can correspond to the number and the intensity of potential arcing events in a DC power system, respectively. The systems and methods can process the PC and PD using one or more arc fault detection algorithms, thereby differentiating between DC arcs and load-switching noise with increased reliability.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part (CIP) of U.S. patentapplication Ser. No. 13/679,039 filed Nov. 16, 2012 entitled SYSTEMS ANDMETHODS OF DISCRIMINATING DC ARCS AND LOAD SWITCHING NOISE.

BACKGROUND

The present application relates generally to detecting arcing inelectrical circuits, and more specifically to systems and methods ofdiscriminating between arcing and load-switching noise in direct current(DC) power systems.

In recent years, DC power systems such as photovoltaic (PV) systems havebeen increasingly employed in home and industrial applications rangingfrom charging batteries to supplying power to the alternating current(AC) grid. Such PV systems can include a plurality of PV modules (e.g.,solar panels) serially connected to form one or more PV strings.Multiple PV strings can be connected in parallel, and routed through acombiner box for ultimately driving a charger or inverter load. In atypical PV system, each PV module can be configured to generate acurrent output of up to about 10 amps at 50 Vdc, and each PV string canbe configured to produce a voltage output of up to about 1000 Vdc ormore, depending on the number of PV modules connected on the PV string.Further, the PV strings connected in parallel can be configured to boostthe total current output of the typical PV system up to about 200 ampsor more.

Because DC power systems such as the PV systems described above can beconfigured to generate relatively high current and voltage outputs,there is a need for systems and methods of detecting arcing in suchpower systems. For example, in the typical PV system, in which thecurrent output and the voltage output can be on the order of 200 ampsand 1000 Vdc, respectively, series arcing can be produced bydisconnecting PV power cables, parallel arcing can be produced byshorting the PV power cables, and ground fault arcing can be produced byshorting the PV power cables to ground. However, known systems andmethods of detecting arcing in power systems have heretofore beengenerally incapable of differentiating between series arcing, parallelarcing, ground fault arcing, etc., and noise generated by charger loads,inverter loads, DC-to-DC load-switching, DC-to-AC load-switching, DCdisconnect switches, radio frequency (RF) pickup, DC power linecommunications, etc., with a high level of reliability.

SUMMARY

In accordance with the present application, systems and methods ofdetecting arcing in a DC power system are disclosed that candifferentiate between DC arcs and load-switching noise with increasedreliability. One such system for detecting arcing in a DC power systemincludes a plurality of current sensors operative to monitor a pluralityof current outputs, respectively, provided over a home run cable, or anyother suitable wiring run. The plurality of current sensors areconfigured and arranged in parallel, series, or any suitable combinationof parallel/series interconnection to provide a combined current outputsignal. The system further includes a rectifier, a filter, a comparator,a pulse integrator, and a processor. The plurality of current sensorsmonitor the plurality of current outputs, respectively, of the DC powersystem, such as a photovoltaic (PV) system, and provide the combinedcurrent output signal that contains high frequency AC currentinformation representing one or more significant di/dt events, which maypotentially be indicative of one or more arcing events. The rectifierreceives the combined current output signal containing the AC currentinformation from the current sensor, and provides a rectified version ofthe combined current output signal to the filter for subsequentfiltering. It is noted that the combined current output signalcontaining the AC current information may alternatively be filteredbefore being rectified. The comparator receives the filtered signal,and, in response to the potential arcing events, generates one or morepulses. The pulse integrator receives the pulses from the comparator,and generates an output indicative of the duration of the respectivepulses. The processor also receives the pulses from the comparator, anddetermines, within a plurality of predetermined time intervals, thepulse count (PC) per predetermined time interval, which can correspondto the number of potential arcing events. The processor further receivesthe output generated by the pulse integrator, and determines, within therespective predetermined time intervals, the pulse duration (PD) perpredetermined time interval, which can correspond to the intensity ofthe respective potential arcing events. The processor then processes thePC and PD, using one or more arc fault detection algorithms, to betterdifferentiate between DC arcs and load-switching noise.

In one aspect, the processor calculates the values of two variables,namely, the average pulse count (APC) and the average pulse durationfluctuation (APDF), at the end of each predetermined time interval. Forexample, the processor can calculate the pulse duration fluctuation(PDF) at the end of each predetermined time interval by taking theabsolute value of the difference between the PD for the most recent timeinterval, and the PD for the time interval occurring one, two, or moretime intervals prior to the most recent time interval, or by any othersuitable technique. The processor further determines, at the end of eachtime interval, whether the ratio, APDF/APC, exceeds a first specifiedthreshold value. In the event the ratio, APDF/APC, exceeds the firstspecified threshold value at the end of a respective time interval, thenthat time interval is deemed to be an interval during which an actualarcing event may have occurred. For example, if the processor determinesthat the ratio, APDF/APC, exceeds the first specified threshold valueduring the respective time interval, then it can generate an output of“1”, or any other suitable output; otherwise, the processor can generatean output of “0”, or any other suitable output. The processor averagesthe outputs (1 or 0) generated over the plurality of time intervals,and, if the average of the respective outputs exceeds a second specifiedthreshold value, then it is assumed that actual arcing has likelyoccurred, and the processor generates another output indicative of sucharcing. In this way, the processor can evaluate the PDF over multipletime intervals, and, if it determines that the PDF over the multipletime intervals is high, then the processor can generate the outputindicating that actual arcing has likely occurred.

In another aspect, the processor calculates the values of threevariables, namely, the APC, the APDF, and the average pulse durationmodulation (APDM), at the end of each predetermined time interval. Forexample, the processor can calculate the APDM by taking four PDmeasurements, PD1, PD2, PD3, PD4, during each predetermined timeinterval, spaced one quarter of the time interval apart, and calculatingthe APDM at the end of each time interval, as follows,APDM=|APD1+APD2−APD3−APD4|+|APD1−APD2−APD3+APD4 |,in which “APD1” is the average of the “PD1” measurements, “APD2” is theaverage of the “PD2” measurements, “APD3” is the average of the “PD3”measurements, and “APD4” is the average of the “PD4” measurements, overthe plurality of predetermined time intervals, or by any other suitabletechnique. The processor further determines, at the end of each timeinterval, whether the ratio, APDF/APC, exceeds a first specifiedthreshold value, and whether the ratio, APDF/APDM, exceeds a secondspecified threshold. In the event it is determined that, at the end of arespective time interval, the ratio, APDF/APC, exceeds the firstspecified threshold value, and the ratio, APDF/APDM, exceeds the secondspecified threshold, then that time interval is deemed to be an intervalduring which an actual arcing event may have occurred. For example, ifthe processor determines that the ratio, APDF/APC, exceeds the firstspecified threshold value and the ratio, APDF/APDM, exceeds the secondspecified threshold value during the respective time interval, then itcan generate an output of “1”, or any other suitable output; otherwise,the processor can generate an output of “0”, or any other suitableoutput. The processor averages the outputs (1 or 0) generated over theplurality of time intervals, and, if the average of the respectiveoutputs exceeds a third specified threshold value, then it is assumedthat actual arcing has likely occurred, and the processor generatesanother output indicative of such arcing. In this way, the processor canmore reliably distinguish between actual arcing and very noisy loads,such as grid-tied inverter loads.

In a further aspect, the processor calculates the values of fivevariables, namely, the APC, the APDF, the APDM, the average pulseduration (APD), and the average pulse count fluctuation (APCF), at theend of each predetermined time interval. For example, the processor cancalculate the APCF by taking the absolute value of the differencebetween the PC for the most recent time interval, and the PC for thetime interval occurring one, two, or more intervals prior to the mostrecent time interval, or by any other suitable technique. The processorfurther determines, at the end of each time interval, whether the ratio,APDF/APC, exceeds a first specified threshold value, whether the ratio,APDF/APDM, exceeds a second specified threshold, whether the ratio,APCF/APC, exceeds a third specified threshold, whether the ratio,APDF/APD, exceeds a fourth specified threshold, whether APC exceeds afifth specified threshold, and whether APD exceeds a sixth specifiedthreshold. In the event it is determined that, at the end of arespective time interval, the ratio, APDF/APC, exceeds the firstspecified threshold value, the ratio, APDF/APDM, exceeds the secondspecified threshold, the ratio, APCF/APC, exceeds the third specifiedthreshold, the ratio, APDF/APD, exceeds the fourth specified threshold,APC exceeds the fifth specified threshold, and APD exceeds the sixthspecified threshold, then that time interval is deemed to be an intervalduring which an actual arcing event may have occurred. The processor cantherefore generate an output of “1”, or any other suitable output;otherwise, the processor can generate an output of “0”, or any othersuitable output. The processor averages the outputs (1 or 0) generatedover the plurality of time intervals, and, if the average of therespective outputs exceeds a seventh specified threshold value, then itis assumed that actual arcing has occurred, and the processor generatesanother output indicative of such arcing. In this way, it can be assuredthat, in the DC power system, there exists some minimal level ofnormalized average fluctuation that is indicative of arcing versus loadnoise.

By determining, within a plurality of predetermined time intervals, atleast the pulse count (PC) per predetermined time interval, and thepulse duration (PD) per predetermined time interval, in which the PC andthe PD can correspond to the number and the intensity of potentialarcing events in a DC power system, respectively, and then processingthe PC and the PD using one or more arc fault detection algorithms, thedisclosed systems and methods of detecting arcing in DC power systemscan differentiate between DC arcs and load-switching noise withincreased reliability.

Other features, functions, and aspects of the invention will be evidentfrom the Detailed Description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one or more embodiments describedherein and, together with the Detailed Description, explain theseembodiments. In the drawings:

FIG. 1a illustrates a block diagram of a typical photovoltaic (PV)system;

FIG. 1b illustrates the PV system of FIG. 1a , further indicatingpossible locations of various types of arcing;

FIG. 1c illustrates the PV system of FIG. 1a , further indicatingpossible sites of arc fault detectors for detecting the various types ofarcing indicated in FIG. 1 b;

FIG. 2 illustrates a block diagram of an exemplary system for detectingarcing in DC power systems, in accordance with the present application;

FIG. 3a illustrates a flow diagram of a first exemplary method ofdetecting arcing in DC power systems, using the system of FIG. 2;

FIG. 3b illustrates a flow diagram of a second exemplary method ofdetecting arcing in DC power systems, using the system of FIG. 2;

FIG. 3c illustrates a flow diagram of a third exemplary method ofdetecting arcing in DC power systems, using the system of FIG. 2;

FIGS. 4a-4d illustrate diagrams of exemplary pulse stream data that canbe generated during startup of an inverter load, and during arcing inthe presence of continuous inverter noise, using the system of FIG. 2;

FIGS. 5a-5c illustrate diagrams of exemplary variables that can bemeasured and calculated as a function of a number of time intervals,using the system of FIG. 2;

FIGS. 6a-6c illustrate diagrams of exemplary techniques fordiscriminating DC arcs from load-switching noise, using the system ofFIG. 2; and

FIG. 7 illustrates a block diagram of an exemplary system for detectingseries arcs in DC power systems, in accordance with the presentapplication.

DETAILED DESCRIPTION

FIG. 1a depicts a typical DC power system, specifically, a photovoltaic(PV) system 100. Such PV systems have been increasingly employed in homeand industrial applications ranging from charging batteries to supplyingpower to the AC grid. The PV system 100 includes a plurality of PVmodules (e.g., solar panels) 101.1-101.n, 103.1-103.m, 105.1-105.p, acombiner box 104, and a load 106. As shown in FIG. 1a , the PV modules101.1-101.n are serially connected to form a first PV string 102.1, thePV modules 103.1-103.n are serially connected to form a second PV string102.2, and the PV modules 105.1-105.n are serially connected to form athird PV string 102.3. Further, the first, second, and third PV strings102.1, 102.2, 102.3 can be connected in parallel, and can be routedthrough the combiner box 104 for ultimately driving the load 106, whichcan be a charger load, an inverter load, or any other suitable load. Asfurther shown in FIG. 1a , the combiner box 104 can include a stringfuse 108 for each PV string, and a surge protector 110. The PV system100 can also include a DC disconnect switch 112. It is noted that the PVsystem 100 may alternatively be configured to include any other suitablenumber of PV modules serially connected to form any other suitablenumber of PV strings.

FIG. 1b depicts a number of exemplary locations 121-129 within the PVsystem 100 where arcing may potentially occur. For example, seriesarcing may potentially occur at the locations 121, 125, 129, parallelarcing may potentially occur at the locations 122, 126, and ground faultarcing may potentially occur at the locations 123, 124, 127, 128.Moreover, FIG. 1c depicts several exemplary sites within the PV system100 where arc fault detectors (AFD) 132, 134, 136 may be situated todetect such potential arcing. For example, the AFDs 132, 134 may besituated within the combiner box 104 where the PV strings are combined,and the AFD 136 may be situated in proximity to the load 106. It isnoted that any other suitable number of AFDs may be employed to detectarcing at any other suitable sites within the PV system 100.

FIG. 2 depicts an exemplary system 200 for detecting arcing in DC powersystems, in accordance with the present application. For example, thesystem 200 may be implemented within one or more AFDs, such as the AFDs132, 134, 136 within the PV system 100, to differentiate between DC arcsand load-switching noise with increased reliability. As shown in FIG. 2,the system 200 includes a current sensor 202, a rectifier 204, a filter206, a comparator 208, a pulse integrator 210, and a processor 212. Thecurrent sensor 202 can be implemented as a current transformer formonitoring a current output of the DC power system. For example, thecurrent sensor 202 implemented as a current transformer can be connectedin series with either the positive (+) DC power line or the negative (−)DC power line. The current sensor 202 provides a signal that containshigh frequency AC current information representing one or moresignificant di/dt events, which may be indicative of one or morepotential arcing events. The rectifier 204, which can be implemented asa full-wave rectifier, receives the signal containing the AC currentinformation from the current sensor 202, and provides a full-waverectified version of the signal to the filter 206 for subsequenthigh-pass filtering.

The comparator 208 receives the filtered signal, and, in response to thepotential arcing events, generates one or more pulses on a line 214. Thepulse integrator 210 receives the pulses from the comparator 208, andgenerates an output indicative of the duration of the respective pulseson a line 216. The processor 212, which can be implemented as amicrocontroller, also receives the pulses from the comparator on theline 214. The processor 212 determines, within a plurality ofpredetermined time intervals, the pulse count (PC) per time interval,which can correspond to the number of potential arcing events. Theprocessor 212 further receives the output generated by the pulseintegrator 210 on the line 216, and determines, within the respectivepredetermined time intervals, the pulse duration (PD) per time interval,which can correspond to the intensity of the respective potential arcingevents. Using one or more arc fault detection algorithms, as furtherdescribed herein, the processor 212 then processes at least the PC andPD to better differentiate between DC arcs and load-switching noise, andgenerates, at least at some times, an arc fault indication 218 as anoutput.

A first exemplary method 300 a of detecting arcing in DC power systemsis described below with reference to FIG. 3a , as well as FIG. 2. Usingthe method 300 a, the system 200 can evaluate the fluctuation of the PDover multiple predetermined time intervals, and, if it determines thatthe fluctuation of the PD over the respective time intervals is high,then the system 200 can generate the arc fault indication 218, therebyindicating that actual arcing has likely occurred. For example, eachpredetermined time interval may be equal to any suitable time interval.In some embodiments, the predetermined time interval can be equal toabout one-half of the AC grid cycle period in order to minimizeelectromagnetic coupling to AC power lines, as well as reduce theswitching noise generated by grid-tied inverter loads. As depicted instep 302, the processor 212 calculates the values of two variables,namely, the average pulse count (APC) and the average pulse durationfluctuation (APDF), at the end of each time interval. For example, suchaveraging may be accomplished using a first-order, low-pass filter toretain the variable values without requiring excessive memory. It isnoted that, because so-called sputtering arcs can be fewer in number butmore intense than continuous arcing, such averaging allows sputteringarcs, as well as more continuous arcing, to be better discriminated fromother noise sources. The time constant of such averaging can range fromabout 20 msecs to 200 msecs, or any other suitable range of time values.

The processor 212 can calculate the pulse duration fluctuation (PDF) atthe end of each predetermined time interval by taking the absolute valueof the difference between the PD for the most recent time interval, andthe PD for the time interval occurring one, two, or more time intervalsprior to the most recent time interval, or by any other suitabletechnique. As depicted in step 304, the processor 212 determines, at theend of each time interval, whether the ratio, APDF/APC, exceeds a firstspecified threshold value, C1. In the event the ratio, APDF/APC, exceedsthe first specified threshold value, C1, at the end of a respective timeinterval, then that time interval is deemed to be an interval duringwhich an actual arcing event may have occurred. For example, if theprocessor 212 determines that the ratio, APDF/APC, exceeds the firstspecified threshold value, C1, during the respective time interval, thenit can generate an output of “1”, or any other suitable output, asdepicted in step 306. Otherwise, the processor 212 can generate anoutput of “0”, or any other suitable output, as depicted in step 308. Asdepicted in step 310, the processor 212 averages the outputs (1 and/or0) generated over the plurality of predetermined time intervals. Forexample, such averaging can be performed using a low-pass filter, arunning sum, or an event counter, over multiple time intervals, or usingany other suitable technique. Further, the time constant of thisaveraging may be in the range of 0.1 secs to 1.0 sec to allow anypossible arc fault indications to occur within a reasonable time. Asdepicted in step 312, the processor 212 then determines whether theaverage of the respective outputs exceeds a specified threshold outputvalue, C0. If the average of the respective outputs exceeds thespecified threshold output value, C0, then it is assumed that actualarcing has occurred, and the processor 212 generates the arc faultindication 218, as depicted in step 314. Otherwise, the method 300 aloops back to step 302.

A second exemplary method 300 b of detecting arcing in DC power systemsis described below with reference to FIG. 3b , as well as FIG. 2. Usingthe method 300 b, the system 200 can more reliably distinguish betweenactual arcing and very noisy loads, such as grid-tied inverter loads. Asdepicted in step 316, the processor 212 calculates the values of threevariables, namely, the APC, the APDF, and the average pulse durationmodulation (APDM), at the end of each predetermined time interval. Forexample, the processor 212 can calculate the APDM by taking four PDmeasurements, PD1, PD2, PD3, PD4, during each time interval, spaced onequarter of the time interval apart, and calculating the APDM at the endof each time interval, as follows,APDM=|APD1+APD2−APD3−APD4|+|APD1−APD2−APD3+APD4|,   (1)in which “APD1” is the average of the respective “PD1” measurements,“APD2” is the average of the respective “PD2” measurements, “APD3” isthe average of the respective “PD3” measurements, and “APD4” is theaverage of the respective “PD4” measurements, over the plurality of timeintervals, or by any other suitable technique. For example, suchaveraging of each quarter-interval measurement can be performed using alow-pass filter over multiple time intervals. Further, each timeinterval can be at or near one-half of the AC grid cycle period, e.g.,1/(2*55 Hz) for 60 Hz or 50 Hz AC grids. As depicted in step 318, theprocessor 212 determines, at the end of each time interval, whether theratio, APDF/APC, exceeds a first specified threshold value, C1, andwhether the ratio, APDF/APDM, exceeds a second specified threshold, C2.In the event it is determined that, at the end of a respective timeinterval, the ratio, APDF/APC, exceeds the first specified thresholdvalue, C1, and the ratio, APDF/APDM, exceeds the second specifiedthreshold, C2, then that time interval is deemed to be an intervalduring which an actual arcing event may have occurred. For example, ifthe processor 212 determines that the ratio, APDF/APC, exceeds the firstspecified threshold value, C1, and the ratio, APDF/APDM, exceeds thesecond specified threshold value, C2, during the respective timeinterval, then it can generate an output of “1”, or any other suitableoutput, as depicted in step 320. Otherwise, the processor 212 cangenerate an output of “0”, or any other suitable output, as depicted instep 322. As depicted in step 324, the processor 212 averages theoutputs (1 and/or 0) generated over the plurality of time intervals. Asdepicted in step 326, the processor 212 then determines whether theaverage of the respective outputs exceeds a specified threshold outputvalue, C0. If the average of the respective outputs exceeds thespecified threshold output value, C0, then it is assumed that actualarcing has occurred, and the processor 212 generates the arc faultindication 218, as depicted in step 328. Otherwise, the method 300 bloops back to step 316.

A third exemplary method 300 c of detecting arcing in DC power systemsis described below with reference to FIG. 3c , as well as FIG. 2. Themethod 300 c provides a way of assuring that, in a DC power system,there exists some minimal level of normalized average fluctuation thatis indicative of arcing versus load noise. As depicted in step 330, theprocessor 212 calculates the values of five variables, namely, the APC,the APDF, the APDM, the average pulse duration (APD), and the averagepulse count fluctuation (APCF), at the end of each predetermined timeinterval. For example, the processor 212 can calculate the APCF bytaking the absolute value of the difference between the PC for the mostrecent time interval, and the PC for the time interval occurring one ortwo intervals prior to the most recent time interval, or by any othersuitable technique. As depicted in step 332, the processor 212determines, at the end of each time interval, whether the ratio,APDF/APC, exceeds a first specified threshold value, C1, whether theratio, APDF/APDM, exceeds a second specified threshold value, C2,whether the ratio, APCF/APC, exceeds a third specified threshold valueC3, whether the ratio, APDF/APD, exceeds a fourth specified thresholdvalue, C4, whether APC exceeds a fifth specified threshold value, C5,and whether APD exceeds a sixth specified threshold value, C6. In theevent it is determined that, at the end of a respective time interval,the ratio, APDF/APC, exceeds the first specified threshold value, C1,the ratio, APDF/APDM, exceeds the second specified threshold value, C2,the ratio, APCF/APC, exceeds the third specified threshold value, C3,the ratio, APDF/APD, exceeds the fourth specified threshold value, C4,APC exceeds the fifth specified threshold value, C5, and APD exceeds thesixth specified threshold value, C6, then that time interval is deemedto be an interval during which an actual arcing event may have occurred.The processor 212 therefore generates an output of “1”, or any othersuitable output, as depicted in step 334. Otherwise, the processor 212generates an output of “0”, or any other suitable output, as depicted instep 336. As depicted in step 338, the processor 212 averages theoutputs (1 and/or 0) generated over the plurality of time intervals. Asdepicted in step 340, the processor 212 then determines whether theaverage of the respective outputs exceeds a specified threshold outputvalue, C0. If the average of the respective outputs exceeds thespecified threshold output value, C0, then it is assumed that actualarcing has occurred, and the processor 212 generates the arc faultindication 218, as depicted in step 342. Otherwise, the method 300 cloops back to step 330.

The disclosed systems and methods of detecting arcing in DC powersystems are further described below with reference to the followingillustrative examples, and FIGS. 1, 2, 3 a, 3 b, 4 a-4 d, 5 a-5 c, and 6a-6 c. In a first example, it is demonstrated that series arcing can bea cause of both pulse duration fluctuation (PDF) and pulse durationmodulation (PDM) in a DC power system. FIG. 4a illustrates an exemplarypulse stream 400 generated by the comparator 208 on the line 214 duringstartup of the load 106, which can be an inverter load. In this firstexample, such an inverter load can be tied to the AC grid, which canhave a frequency equal to about 60 Hz. As shown in FIG. 4a , the pulsestream 400 includes a series of pulse bursts 401, 402, 403, 404, whichgenerally represent load-switching noise produced during startup of theinverter load. Each pulse burst 401, 402, 403, 404 occurs within apredetermined time interval of 1/(2*60 Hz), or about 8,333 μsecs. FIG.4b illustrates a series of exemplary pulses 410, which may be includedin one of the pulse bursts 401, 402, 403, 404. As shown in FIG. 4b ,both the periods and the durations of the respective pulses 410 aregenerally uniform.

FIG. 4c illustrates an exemplary pulse stream 420 generated by thecomparator 208 on the line 214 during series arcing in the presence ofcontinuous inverter load noise. As shown in FIG. 4c , the pulse stream420 includes a series of pulse bursts 411, 412, 413, 414, each occurringwithin the predetermined time interval of about 8,333 μsecs. As furthershown in FIG. 4c , the series arcing has caused an extra pulse burst 416to be introduced between the pulse bursts 412, 413. It is noted that theextra pulse burst 416 caused by the series arcing is not insynchronization with the periodic pulse bursts 411, 412, 413, 414resulting from the load-switching noise. Because the series arcing hasintroduced the extra pulse burst 416 between the pulse bursts 412, 413,such series arcing has caused some PDF per time interval within thepulse stream 420.

FIG. 4d illustrates a series of exemplary pulses 430 that may beincluded in one of the pulse bursts 411, 412, 413, 414. By comparing theseries of pulses 430 generated during series arcing with the series ofpulses 410 generated during startup of the inverter load, it can beobserved that the series arcing has also caused some PDM within theseries of pulses 430. It is noted that the widths of the respectivepulses 430 are generally narrower than the widths of the pulses 410,thereby indicating that the pulses 410 produced during startup of theinverter load can be more intense than the pulses 430 produced duringseries arcing in the presence of continuous inverter load noise.Nonetheless, it can be observed that the general randomness of arcingcan produce greater PDF and/or PDM than the more uniform load-switchingnoise.

In a second example, it is demonstrated that both the pulse count (PC)per time interval, and the pulse duration (PD) per time interval, can begreater during startup of an inverter load than during series arcing inthe presence of continuous inverter noise, and therefore an analysis ofthe measured PC and/or PD per time interval alone may be insufficient toreliably discriminate between DC arcs and load-switching noise. FIG. 5aillustrates pulse counts measured by the processor 212 for a pluralityof exemplary time intervals numbered 5 through 15. As shown in FIG. 5a ,the measured PC per time interval during startup of the inverter load isgenerally within the range of about 20 to 30 counts, whereas themeasured PC per time interval during series arcing is generally withinthe range of about 10 to 20 counts, with the exception of time interval11, in which the measured PC during series arcing is between 20 countsand 30 counts. FIG. 5b illustrates pulse durations measured by theprocessor 212 for the exemplary time intervals numbered 5 through 15. Asshown in FIG. 5b , the measured PD per time interval during startup ofthe inverter load is generally within the range of about 200 to 300μsecs, whereas the measured PD per time interval during series arcing isgenerally within the range of about 0 to 100 μsecs, with the exceptionof time interval 11, in which the measured PD during series arcing isbetween 100 μsecs and 200 μsecs.

FIG. 5c illustrates the pulse duration fluctuation (PDF) calculated bythe processor 212 for the exemplary time intervals numbered 5 through15. As described herein with respect to the first exemplary method 300a, the processor 212 can calculate the PDF at the end of eachpredetermined time interval by taking the absolute value of thedifference between the PD for the most recent time interval, and the PDfor the time interval occurring one, two, or more time intervals priorto the most recent time interval. As shown in FIG. 5c , the calculatedPDF during startup of the inverter load is generally within the range of0 to 10 μsecs, with the exceptions of time intervals 5, 7, 11, and 15,in which the calculated PDF is just over 20 μsecs, about 30 μsecs, justover about 10 μsecs, and between 10 μsecs and 20 μsecs, respectively. Asfurther shown in FIG. 5c , the calculated PDF during series arcing isalso generally within the range of 0 to 10 μsecs, with the exceptions oftime intervals 11, 13, 14, and 15, in which the calculated PDF is justover about 40 μsecs, between 40 μsecs and 50 μsecs, between 10 μsecs and20 μsecs, and about 20 μsecs, respectively. Accordingly, based on thecalculated PDFs illustrated in FIG. 5c , an analysis of the calculatedPDF per time interval alone may also be insufficient to reliablydiscriminate between DC arcs and load-switching noise.

In a third example, it is demonstrated that an analysis of at least theratio, PDF/PC, at the end of each time interval, would be sufficient toreliably discriminate between DC arcs and load-switching noise. FIG. 6aillustrates the ratio, PDF/PC, at the end of each exemplary timeinterval numbered 5 through 15, as determined by the processor 212. Asshown in FIG. 6a , the ratio, PDF/PC, determined during series arcing isgenerally significantly greater than the corresponding ratio, PDF/PC,determined during startup of the inverter load (see, e.g., therespective ratios, PDF/PC, for time intervals 6, and 8 through 15). Itis noted that, if the ratio, APDF/APC, were determined at the end ofeach numbered time interval (as described herein with reference to step304 of the first exemplary method 300 a), then such ratios, APDF/APC,determined during series arcing would also be significantly greater thanthe corresponding ratios, APDF/APC, determined during startup of theinverter load. Moreover, if the processor 212 first calculated the APDFand APC at the end of each numbered time interval, and then determinedthe ratios, APDF/APC, for each numbered time interval, then such ratios,APDF/APC, determined during series arcing would likewise besignificantly greater than the corresponding ratios, APDF/APC,determined during startup of the inverter load. Accordingly, based onthe ratios, PDF/PC, illustrated in FIG. 6a , it can be concluded thatanalyzing at least the ratio, PDF/PC, at the end of each time interval,would be sufficient to discriminate between DC arcs and load-switchingnoise with increased reliability.

In this third example, it is further demonstrated that an analysis of atleast the ratio, PDF/PDM, at the end of each time interval, would alsobe sufficient to reliably discriminate between DC arcs and very noisyloads, such as grid-tied inverter loads. FIG. 6b illustrates the PD perquarter time interval for the exemplary time intervals numbered 5through 15, as measured by the processor 212. As shown in FIG. 6b , themajority of the PD for each time interval occurs within a fraction ofthe respective time interval. For example, for each of the numbered timeintervals, most of the PD, ranging from about 250 μsecs to 300 μsecs,occurs within a fraction of the time interval near the start of therespective interval. Based on the measured PD per quarter time intervalillustrated in FIG. 6b , it can be concluded that, in this thirdexample, there can be significant pulse duration modulation (PDM) atabout twice the grid frequency, e.g., 2×60 Hz or 120 Hz.

FIG. 6c illustrates the ratio, PDF/PDM, determined by the processor 212for the exemplary time intervals numbered 5 through 15. Like thecalculation of the APDM, as described herein with respect to the secondexemplary method 300 b, the PDM can be calculated by taking four PDmeasurements, PD1, PD2, PD3, PD4, during each numbered time interval,spaced one quarter of the time interval apart, and calculating the PDMat the end of each time interval as follows,PDM=|PD1+PD2−PD3−PD4|+|PD1−PD2−PD3+PD4|,   (2)or by any other suitable technique. As shown in FIG. 6c , the ratio,PDF/PDM, determined during series arcing is generally significantlygreater than the corresponding ratio, PDF/PDM, determined during startupof the inverter load (see, e.g., the respective ratios, PDF/PDM, fortime intervals 6, and 8 through 15). It is noted that, if the ratio,APDF/APDM, were determined at the end of each numbered time interval (asdescribed herein with reference to step 318 of the second exemplarymethod 300 b), then such ratios, APDF/APDM, determined during seriesarcing would also be significantly greater than the correspondingratios, APDF/APDM, determined during startup of the inverter load.Moreover, if the processor 212 first calculated the APDF and APDM at theend of each numbered time interval, and then determined the ratios,APDF/APDM, for each numbered time interval, then such ratios, APDF/APDM,determined during series arcing would likewise be significantly greaterthan the corresponding ratios, APDF/APDM, determined during startup ofthe inverter load. Accordingly, based on the ratios, PDF/PDM,illustrated in FIG. 6c , it can be concluded that analyzing at least theratio, PDF/PDM, at the end of each time interval, would be sufficient todiscriminate between DC arcs and load-switching noise with increasedreliability.

Having described the above exemplary embodiments of the disclosedsystems and methods of detecting arcing in DC power systems, otheralternative embodiments or variations may be made. For example, it wasdescribed herein, e.g., with reference to the third exemplary method 300c, that the processor 212 can determine, at the end of a plurality ofpredetermined time intervals, whether the ratio, APDF/APC, exceeds afirst specified threshold value, C1, whether the ratio, APDF/APDM,exceeds a second specified threshold value, C2, whether the ratio,APCF/APC, exceeds a third specified threshold value C3, whether theratio, APDF/APD, exceeds a fourth specified threshold value, C4, whetherAPC exceeds a fifth specified threshold value, C5, and/or whether APDexceeds a sixth specified threshold value, C6. In some embodiments, thedetection of arcing in DC power systems can alternatively be based ondetermining, at the end of the respective time intervals, whether theratio, PDF/PC, exceeds a first specified threshold value, C1, whetherthe ratio, PDF/PDM, exceeds a second specified threshold value, C2,whether the ratio, PCF/PC, exceeds a third specified threshold value C3,whether the ratio, PDF/PD, exceeds a fourth specified threshold value,C4, whether the PC exceeds a fifth specified threshold value, C5, and/orwhether the PD exceeds a sixth specified threshold value, C6.

It was also described herein that the current sensor 202 can beimplemented as a current transformer for monitoring the current outputof a DC power system. In some embodiments, the current sensor 202 may beimplemented using a DC current sensor, which can also serve as anAC-detecting current transformer with appropriate circuitry. It is notedthat DC current data provided by the DC current sensor can augment thehigh frequency AC current information to improve arc fault detectionrelative to power system noise. Moreover, in some embodiments, thevoltage output of a DC power system may be monitored instead of thecurrent output for detecting arcing in the DC power system. Suchembodiments may employ power system voltage data, as well as powersystem current data, to better discriminate between series arcing andparallel arcing. In addition, in some embodiments, the current sensor202 may be replaced with an AC voltage sensor, which may be connectedacross the DC power lines. Such an AC voltage sensor can be implementedas a capacitor-coupled current transformer for providing electricalisolation between the DC power system and one or more AFDs.

It was also described herein that the PV system 100 can include a DCdisconnect switch 112, which may be employed to extinguish arcing oncesuch arcing has been detected. In some embodiments, in order toextinguish such arcing, PV systems may include a solid-state switchwithin one or more PV modules for disconnecting one or more PV panels,one or more contactors or circuit breakers within a combiner box fordisconnecting one or more PV strings from a load, and/or one or moremechanisms for opening/shorting the load.

It was also described herein that the PV system 100 could include thecombiner box 104. In some embodiments, such a PV system may beimplemented without a combiner box, thereby allowing one or more PVstrings to be directly connected to the load.

In addition, it is noted that the ratios employed in the exemplarymethods 300 a, 300 b, 300 c of detecting arcing in DC power systemsgenerally do not require the use of complex division operations, but canbe performed with conditional testing, e.g., after multiplying thespecified thresholds by the denominators of the respective ratios. It isfurther noted that more complex tests may be defined by combining two ormore of these ratios. For example, the two ratios, APDF/APC andAPDF/APDM, may be combined to form a single ratio, APDF²/APC/APDMLikewise, the two ratios, PDF/PC and PDF/PDM, may be combined to form asingle ratio, PDF²/PC/PDM. Other suitable combinations of these ratiosmay also be employed to better discriminate between DC arcs andload-switching noise.

It is further noted that one or more embodiments described herein caninclude PV loads such as optimizers for adjusting DC-to-DC conversionsso that each PV module or PV string operates at its maximum power point,micro-inverters for adjusting DC-to-AC conversions so that each PVmodule operates at its maximum power point, as well as DC-to-ACinverters connected to one or more PV strings. Embodiments describedherein can also be combined with arc fault detectors (AFDs) attached toeach PV module, and can be used in conjunction with ground faultdetectors.

In addition, it was described herein that the system 200 (see FIG. 2)for detecting arcing in DC power systems could include the currentsensor 202, the rectifier 204, the filter 206, the comparator 208, thepulse integrator 210, and the processor 212, and that the current sensor202 could be implemented as a current transformer for monitoring acurrent output of a DC power system. In some embodiments, a system fordetecting arcing in DC power systems can include a plurality of suchcurrent sensors to provide improved detection of series arcs in a DCpower system.

FIG. 7 depicts an exemplary system 700 for detecting arcing on home runcable, or any other suitable wiring run, in a DC power system. As shownin FIG. 7, the system 700 includes a plurality of current sensors702.1-702.4, a rectifier 704, a filter 706, a comparator 708, a pulseintegrator 710, and a processor 712. For example, the plurality ofcurrent sensors 702.1-702.4 can each be implemented as a currenttransformer for monitoring a current output of the DC power system. Theoperation of the system 700 is like that of the system 200 (see FIG. 2)with the exception that the system 700 operates using the four currentsensors 702.1-702.4. It is noted that FIG. 7 depicts the system 700including the four current sensors 702.1-702.4 for purposes ofillustration, and that the system 700 may alternatively include anyother suitable number of such current sensors.

The system 700 will be further understood with reference to thefollowing illustrative example and FIG. 7. In this example, the fourcurrent sensors 702.1-702.4 are connected in parallel to monitor four PVstrings, respectively, on a home run cable that can include a total oftwelve or any other suitable number of such PV strings. Further, in thisexample, it is assumed that series arcing occurs on the home run cablewith about 3 amps (AC) of current fluctuation. Assuming the sameimpedance, each of the twelve PV strings on the home run cable thereforereceives an arc current of about 3/12 or 0.25 amps (AC). With regard tothe four PV strings monitored by the respective current sensors702.1-702.4, a first PV string monitored by the current sensor 702.1receives an arc current, I₁, of about 0.25 amps (AC), a second PV stringmonitored by the current sensor 702.2 receives an arc current, I₂, ofabout 0.25 amps (AC), a third PV string monitored by the current sensor702.3 receives an arc current, I₃, of about 0.25 amps (AC), and a fourthPV string monitored by the current sensor 702.4 receives an arc current,I₄, of about 0.25 amps (AC). However, because the four current sensors702.1-702.4 are connected in parallel, the combined arc current providedto the full-wave rectifier 704 is about 4*0.25 or 1 amp (AC).

By employing the four current sensors 702.1-702.4 connected in parallelto provide an increased level of arc current to the full-wave rectifier704 within the system 700 (see FIG. 7), improved detection of seriesarcs on home run cable, or any other suitable wiring run, can beachieved. It is noted that, although FIG. 7 depicts the four currentsensors 702.1-702.4 connected in parallel, any suitable number of suchcurrent sensors may be connected in parallel, in series, or in any othersuitable combination of parallel/series interconnection. It is furthernoted that such series arcing having about 3 amps (AC) of currentfluctuation is discussed above for purposes of illustration, and thatthe system 700 may be employed to detect any other suitable level ofseries arcing.

It will be apparent that one or more embodiments described herein may beimplemented in many different forms of software and/or hardware. Forexample, one or more embodiments described herein may include suitableconfigurations of one or more computerized devices, hardware processors,and/or the like to carry out and/or support any or all of the systemsand/or methods described herein. Further, one or more computerizeddevices, processors, digital signal processors, etc., may be programmedand/or configured to implement the systems and methods described herein.

It will be appreciated by those of ordinary skill in the art thatfurther modifications to and variations of the above-described systemsand methods of detecting arcing in DC power systems may be made withoutdeparting from the inventive concepts disclosed herein. Accordingly, theinvention should not be viewed as limited except as by the scope andspirit of the appended claims.

What is claimed is:
 1. A method of detecting arcing in a DC powersystem, the DC power system including a plurality of current outputsprovided over a home run cable and connected to a load, the methodcomprising: providing an arc fault detector including a plurality ofcurrent sensors, a comparator, and a processor; connecting the pluralityof current sensors to the plurality of current outputs, sensing aplurality of currents flowing in the DC power system at the respectivecurrent outputs; determining, by the plurality of current sensors, aplurality of current signals related to the plurality of currents,respectively, flowing in the DC power system; combining the plurality ofcurrent signals to provide a combined current signal to the comparator;in response to at least one change in the combined current signal overtime (di/dt), generating one or more pulses by the comparator, therespective pulses having associated durations; processing, by theprocessor, the respective pulses to determine a presence of arcing inthe DC power system, the processing of the respective pulses including:counting the number of pulses in at least one predetermined timeinterval to provide a pulse count for the predetermined time interval;measuring a fluctuation in the durations of the respective pulses in thepredetermined time interval; calculating a first ratio of thefluctuation in the durations of the respective pulses to the pulsecount; and determining the presence of arcing in the DC power systembased at least in part on the first ratio; and having determined thepresence of arcing in the DC power system, disconnecting, by a DCdisconnect switch, the plurality of current outputs from the load inorder to extinguish the arcing in the DC power system.
 2. The method ofclaim 1 wherein the determining of the presence of arcing includesdetermining that the first ratio exceeds a first specified thresholdvalue.
 3. The method of claim 1 wherein the counting of the number ofpulses includes counting the number of pulses in each of a plurality ofpredetermined time intervals to provide pulse counts for the respectivepredetermined time intervals, and wherein the measuring of thefluctuation in the durations of the respective pulses includes measuringfluctuations in the durations of the respective pulses in the respectivepredetermined time intervals.
 4. The method of claim 3 wherein thecalculating of the first ratio includes calculating an average of thepulse counts after each predetermined time interval, calculating anaverage of the fluctuations in the durations of the respective pulsesafter each predetermined time interval, and calculating the first ratiobased on the average of the fluctuations in the durations of therespective pulses and the average of the pulse counts.
 5. The method ofclaim 1 further comprising: measuring, by the processor, a modulation ofthe durations of the respective pulses in the predetermined timeinterval.
 6. The method of claim 5 further comprising: calculating, bythe processor, a second ratio of the fluctuation in the durations of therespective pulses to the modulation of the durations of the respectivepulses.
 7. The method of claim 6 wherein the determining of the presenceof arcing includes determining the presence of arcing in the DC powersystem based at least in part on the first ratio and the second ratio.8. The method of claim 7 wherein the determining of the presence ofarcing includes determining that the first ratio exceeds a firstspecified threshold value, and determining that the second ratio exceedsa second specified threshold value.
 9. The method of claim 6 wherein themeasuring of the fluctuation in the durations of the respective pulsesincludes measuring fluctuations in the durations of the respectivepulses for a plurality of predetermined time intervals, and wherein themeasuring of the modulation of the durations of the respective pulsesincludes measuring modulations of the durations of the respective pulsesfor the plurality of predetermined time intervals.
 10. The method ofclaim 9 wherein the calculating of the second ratio includes calculatingan average of the fluctuations in the durations of the respective pulsesafter each predetermined time interval, calculating an average of themodulations of the durations of the respective pulses after eachpredetermined time interval, and calculating the second ratio based onthe average of the fluctuations in the durations of the respectivepulses and the average of the modulations of the durations of therespective pulses.
 11. The method of claim 6 further comprising:measuring, by the processor, a fluctuation in the pulse count for thepredetermined time interval.
 12. The method of claim 11 furthercomprising: calculating, by the processor, one or more of a third ratioof the fluctuation in the pulse count to the pulse count, and a fourthratio of the fluctuation in the durations of the respective pulses tothe durations of the respective pulses.
 13. The method of claim 12wherein the determining of the presence of arcing includes determiningthe presence of arcing in the DC power system based at least in part onthe first ratio, and one or more of the second ratio, the third ratio,and the fourth ratio.
 14. The method of claim 13 wherein the determiningof the presence of arcing includes determining that the first ratioexceeds a first specified threshold value, determining that the secondratio exceeds a second specified threshold value, determining that thethird ratio exceeds a third specified threshold value, and determiningthat the fourth ratio exceeds a fourth specified threshold value. 15.The method of claim 14 wherein the determining of the presence of arcingfurther includes determining that the pulse count exceeds a fifthspecified threshold value, and determining that the durations of therespective pulses exceed a sixth specified threshold value.
 16. Themethod of claim 15 wherein the measuring of the fluctuation in the pulsecount includes measuring fluctuations in the pulse counts for aplurality of predetermined time intervals, and wherein the measuring ofthe fluctuation of the durations of the respective pulses includesmeasuring fluctuations of the durations of the respective pulses for theplurality of predetermined time intervals.
 17. The method of claim 16wherein the calculating of the third ratio includes calculating anaverage of the fluctuations in the pulse counts after each predeterminedtime interval, calculating an average of the pulse counts after eachpredetermined time interval, and calculating the third ratio based onthe average of the fluctuations in the pulse counts and the average ofthe pulse counts.
 18. The method of claim 16 wherein the calculating ofthe fourth ratio includes calculating an average of the fluctuations ofthe durations of the respective pulses after each predetermined timeinterval, calculating an average of the durations of the respectivepulses after each predetermined time interval, and calculating thefourth ratio based on the average of the fluctuations of the durationsof the respective pulses and the average of the durations of therespective pulses.
 19. A system for detecting arcing in a DC powersystem, the DC power system including a plurality of current outputsprovided over a home run cable and connected to a load, a plurality ofcurrents flowing in the DC power system at the respective currentoutputs, the system comprising: a plurality of current sensorsconnectable to the plurality of current outputs, respectively, whereinthe plurality of current sensors are operative to determine a pluralityof current signals related to the plurality of currents, respectively,flowing in the DC power system, and wherein the plurality of currentsensors are configured and arranged to combine the plurality of currentsignals in order to provide a combined current signal; a comparatoroperative to generate one or more pulses in response to at least onechange in the combined current signal over time (di/dt), the respectivepulses having associated durations; a processor operative: to processthe respective pulses to determine a presence of arcing in the DC powersystem by: counting the number of pulses in at least one predeterminedtime interval to provide a pulse count for the predetermined timeinterval; measuring a fluctuation in the durations of the respectivepulses in the predetermined time interval; calculating a first ratio ofthe fluctuation in the durations of the respective pulses to the pulsecount; and determining the presence of arcing in the DC power systembased at least in part on the first ratio; and a DC disconnect switchoperative to disconnect the plurality of current outputs from the loadin order to extinguish the arcing in the DC power system.